*2.6. Effect of Coatings on Soluble Solid Concentration and pH*

As shown in Figure 7, the total soluble solid (TSS), measured as <sup>o</sup>Brix value, increased in all samples during the storage period. The increase in TSS is due to the conversion of starch and non-starch polysaccharides to simple sugar by hydrolytic processes [27]. After 16 days of storage, TSS was highest in CON (21.3 <sup>o</sup>Brix) and lowest in ChCSA (14.3 <sup>o</sup>Brix). Chitosan- and alginate-based coatings were observed to inhibit metabolic and hydrolytic reactions associated with TSS increase in various fruits, including Chinese winter jujube, longan, and fig fruits [27,40–42].Thus, low TSS in ChCSA-coated samples could be attributed to the effective combination of Ch and SA, which reduced metabolic reactions and retarded polysaccharides breakdown processes [13].

Noticeable changes in the pH value of samples occurred after 16 days of storage (Figure 8). CON sample showed a sharp decrease (6.5 to 4.9), while coated samples showed marginal pH changes after the storage period. During post-harvest storage, a decrease in pH is typical and attributed to the production of organic acids by respiratory metabolism [34]. Low pH in CON may be related to the utilization of polysaccharide substrates by microorganisms, which led to the increased production of acidic metabolites [6]. Similar marginal changes in pH value were observed for coated nectarine slices [25] and fresh-cut watermelon [43].

ported [33,40].

*2.6. Effect of Coatings on Soluble Solid Concentration and pH*

and retarded polysaccharides breakdown processes [13].

By modifying the gas atmosphere around the fruit tissue, polysaccharide coatings with semipermeable properties on the surface of fruits impede the rate of respiration and ripening during storage, thus retaining the quality attributes of products [39]. Similar gaseous barrier effects of polysaccharide-based coatings on fresh-cut products have been re-

As shown in Figure 7, the total soluble solid (TSS), measured as oBrix value, increased in all samples during the storage period. The increase in TSS is due to the conversion of starch and non-starch polysaccharides to simple sugar by hydrolytic processes [27]. After 16 days of storage, TSS was highest in CON (21.3 oBrix) and lowest in ChCSA (14.3 oBrix). Chitosan- and alginate-based coatings were observed to inhibit metabolic and hydrolytic reactions associated with TSS increase in various fruits, including Chinese winter jujube, longan, and fig fruits [27,40–42].Thus, low TSS in ChCSA-coated samples could be attributed to the effective combination of Ch and SA, which reduced metabolic reactions

**Figure 7.** The effect of layer-by-layer and single-layer coatings on the percentage of oBrix of freshcut purple sweet potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p ≤* 0.05). **Figure 7.** The effect of layer-by-layer and single-layer coatings on the percentage of <sup>o</sup>Brix of fresh-cut purple sweet potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p* ≤ 0.05). *Gels* **2022**, *8*, x FOR PEER REVIEW 8 of 14

**Figure 8.** The effect of single-layer and gel coatings on the pH on fresh-cut purple sweet potato potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p ≤* 0.05). **Figure 8.** The effect of single-layer and gel coatings on the pH on fresh-cut purple sweet potato potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p* ≤ 0.05).

#### *2.7. Effect of Coatings on Total Anthocyanin Content and Total Phenolic Content 2.7. Effect of Coatings on Total Anthocyanin Content and Total Phenolic Content*

Variations in the total anthocyanin content (TAC) were observed in samples during storage, with a more pronounced decrease in CON from 11.1 to 8.4 mg cyanidin-3-glucoside/g after 16 days of storage (Figure 9). These data are consistent with previous studies which showed that anthocyanin content was influenced by the storage time as well as the coating treatment [24]. Moreover, edible coatings have been reported to be beneficial in inhibiting the degradation pathways of anthocyanins in various anthocyanin-rich produce [44–46]. Moreover, variations in TAC during storage according to different edible coatings have been previously observed [27,47]. Variations in the total anthocyanin content (TAC) were observed in samples during storage, with a more pronounced decrease in CON from 11.1 to 8.4 mg cyanidin-3 glucoside/g after 16 days of storage (Figure 9). These data are consistent with previous studies which showed that anthocyanin content was influenced by the storage time as well as the coating treatment [24]. Moreover, edible coatings have been reported to be beneficial in inhibiting the degradation pathways of anthocyanins in various anthocyanin-rich produce [44–46]. Moreover, variations in TAC during storage according to different edible coatings have been previously observed [27,47].

**Figure 9.** The effect of single-layer and gel coatings on the anthocyanin content on fresh-cut purple sweet potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper

Storage Time ( days) 0 4 8 12 16

Cd Ac

BCa

Bb AbAb

Ba

Bb

Cc Cc

Aa Ab

Similar trends were observed for TPC, in which coated samples prevented phenolic compounds oxidation and degradation, having higher TPC values (2.27–3.56 mg GAE/g) compared to uncoated samples (1.41 mg GAE/g) throughout the storage period (Figure 10). Connor et al. [48] reported that several causes of physiological stress could promote

BcBd

BaAb

TAC (mg cyanidin-3-glucoside/g)

5

10

Ca

Cb

Aa

Bb

15

20

25

CON Ch SA+C ChCSA

case) are significantly different (Tukey's HSD Test, *p ≤* 0.05).

**Figure 8.** The effect of single-layer and gel coatings on the pH on fresh-cut purple sweet potato potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case)

Storage Time (days) 0 4 8 12 16

Ac

Aa

Ac AbAc

Ad BcAb Aa Ac Bc Ab AaAd Bc Ab Aa

Variations in the total anthocyanin content (TAC) were observed in samples during storage, with a more pronounced decrease in CON from 11.1 to 8.4 mg cyanidin-3-glucoside/g after 16 days of storage (Figure 9). These data are consistent with previous studies which showed that anthocyanin content was influenced by the storage time as well as the coating treatment [24]. Moreover, edible coatings have been reported to be beneficial in inhibiting the degradation pathways of anthocyanins in various anthocyanin-rich produce [44–46]. Moreover, variations in TAC during storage according to different edible

*2.7. Effect of Coatings on Total Anthocyanin Content and Total Phenolic Content*

are significantly different (Tukey's HSD Test, *p ≤* 0.05).

pH

5

6

7

Bc AbAa

CON Ch SA+C ChCSA

8

coatings have been previously observed [27,47].

**Figure 9.** The effect of single-layer and gel coatings on the anthocyanin content on fresh-cut purple sweet potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p ≤* 0.05). **Figure 9.** The effect of single-layer and gel coatings on the anthocyanin content on fresh-cut purple sweet potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p* ≤ 0.05).

Similar trends were observed for TPC, in which coated samples prevented phenolic compounds oxidation and degradation, having higher TPC values (2.27–3.56 mg GAE/g) compared to uncoated samples (1.41 mg GAE/g) throughout the storage period (Figure 10). Connor et al. [48] reported that several causes of physiological stress could promote Similar trends were observed for TPC, in which coated samples prevented phenolic compounds oxidation and degradation, having higher TPC values (2.27–3.56 mg GAE/g) compared to uncoated samples (1.41 mg GAE/g) throughout the storage period (Figure 10). Connor et al. [48] reported that several causes of physiological stress could promote the enzymatic oxidation of phenolic compounds during storage. However, coatings, especially composite coatings, could be beneficial in alleviating these oxidation processes [49]. Similar to the reports of Kou et al. [27], composite ChCSA-coated samples maintained a higher phenolic content throughout 16 days of storage. Moreover, the increase in the phenolic contents could be explained by the effect of Ch/SA coating in promoting phenylalanine ammonia-lyase (PAL) activity which led to the accumulation of phenolic compounds [27]. *Gels* **2022**, *8*, x FOR PEER REVIEW 9 of 14 the enzymatic oxidation of phenolic compounds during storage. However, coatings, especially composite coatings, could be beneficial in alleviating these oxidation processes [49]. Similar to the reports of Kou et al. [27], composite ChCSA-coated samples maintained a higher phenolic content throughout 16 days of storage. Moreover, the increase in the phenolic contents could be explained by the effect of Ch/SA coating in promoting phenylalanine ammonia-lyase (PAL) activity which led to the accumulation of phenolic compounds [27].

**Figure 10.** The effect of single-layer and gel coatings on the total phenolic content on fresh-cut purple sweet potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p ≤* 0.05). **Figure 10.** The effect of single-layer and gel coatings on the total phenolic content on fresh-cut purple sweet potatoes. Vertical bars represent means and standard deviation. Bars with different alphabets within the same storage day (lower case) or same treatment group at different storage days (upper case) are significantly different (Tukey's HSD Test, *p* ≤ 0.05).

#### **3. Conclusions 3. Conclusions**

**4. Materials and Methods**

*4.2. Coating Solutions Preparation*

*4.1. Materials*

This study examined the effect of chitosan-, sodium alginate-, and their composite gel- coatings on the post harvest quality and shelf life of fresh-cut purple sweet potatoes. This study examined the effect of chitosan-, sodium alginate-, and their composite gelcoatings on the post harvest quality and shelf life of fresh-cut purple sweet potatoes. During

During 16 days of storage, various physiological and biochemical reactions associated with quality deterioration were effectively controlled in coated samples. For instance, im-

with gel coating formed by Ch and SA multilayer solutions in the presence of CaCl2, as a cross linking agent. The observed effects were attributed to enhanced barrier properties and antimicrobial properties, which regulated quality losses by transpiration, respiration, oxidation, and cellular degradation. In summary, ChCSA gel coating achieved the best preservative effect on the post harvest quality and shelf life of fresh-cut purple sweet potatoes, indicating the superiority of layer-by-layer coating over single-layer coating.

The experiments were performed with mature purple flesh sweet potato (PFSP) from a farm in Haenam-gun in Korea. The samples were stored at 5 °C. In addition, sodium alginate (32–250 kDa, Duksan Chemicals, Ansan-si, Republic of Korea), high molecular weight chitosan (≥75% deacetylation, Sigma Aldrich, USA), calcium chloride, glacial acetic

The chitosan solution was prepared according to the method described by [50]. Chitosan powder was mixed with distilled water containing glacial acetic acid (0.5% v/v) at

acid, and Tween-80 were obtained from Sigma Aldrich (St. Louis, MO, USA).

16 days of storage, various physiological and biochemical reactions associated with quality deterioration were effectively controlled in coated samples. For instance, improved quality retention and microbial inhibitions were observed in samples preserved with gel coating formed by Ch and SA multilayer solutions in the presence of CaCl2, as a cross linking agent. The observed effects were attributed to enhanced barrier properties and antimicrobial properties, which regulated quality losses by transpiration, respiration, oxidation, and cellular degradation. In summary, ChCSA gel coating achieved the best preservative effect on the post harvest quality and shelf life of fresh-cut purple sweet potatoes, indicating the superiority of layer-by-layer coating over single-layer coating.
